Hydraulic flushing (HF) is an efficient technique for improving coal mine gas extraction. Although much research has been conducted in recent decades, many problems on its mechanism of enhancing permeability still exist. For example, how does coal strength affect the enhanced permeability by HF? Can a larger borehole generate a higher gas flowrate? Therefore, a coupled gas flow-geomechanics model was developed based on an equivalent fractured-coal model to solve these issues. Plastic volumetric strain was introduced in this model to quantify the coal structure, which can improve the numerical models for coal permeability, gas flow in fracture and diffusion in matrix, respectively. The model was used to describe gas flow in the virgin and disturbed coal seams. Results show that neglecting coal failure can cause erroneous stress and permeability distributions, and lead to an overestimation of gas pressure and an underestimation of gas extraction amount. Effects of coal strength and borehole diameter on the distributions of stress, plastic zone, permeability and gas pressure were systematically studied. It indicates that the stress relief, plastic and enhanced-permeability zones around the boreholes in soft coal are obviously larger than those in hard coal. The stress-concentration level and permeability reduction in soft coal are greater than those in hard coal. After 500 d extraction, gas pressure in soft coal has a higher decrease than that in hard coal, indicating that the enhanced permeability by HF of soft coal is larger than that of hard coal. Besides, the investigation on the effects of borehole diameter shows that there exists an optimal borehole diameter. Gas pressure between the boreholes decreases before the optimal diameter, and increases sharply after the optimal diameter due to the high stress concentration. For the Pingmei 8th mine, the optimal borehole diameter is about 1.0 m. Field tests indicate that the optimized HF can considerably eliminate the risk of coal and gas outburst.

水力冲洗（HF）是提高煤矿瓦斯抽采效率的有效技术。尽管最近几十年来进行了大量研究，但是关于其增强渗透性的机理仍然存在许多问题。例如，煤炭强度如何影响HF增强的渗透性？更大的钻孔能产生更高的气体流量吗？因此，建立了基于等效裂隙煤模型的瓦斯流动-地质力学耦合模型来解决这些问题。在该模型中引入塑性体积应变来量化煤的结构，这可以分别改善煤渗透率，裂缝中的瓦斯流量和基质中扩散的数值模型。该模型用于描述原始气流和扰动气流煤层。结果表明，忽略煤的破坏会引起错误的应力和渗透率分布，并导致瓦斯压力的高估和瓦斯抽采量的低估。系统地研究了煤强度和井眼直径对应力，塑性区，渗透率和气压分布的影响。结果表明，软煤井孔周围的应力释放区，塑性区和增渗区明显大于硬煤区。软煤的应力集中水平和渗透率降低大于硬煤。抽提500 d后，软煤的气压下降比硬煤的气压下降高，这表明软煤的HF增强的渗透性大于硬煤。除了，对井眼直径影响的研究表明，存在最佳井眼直径。由于高应力集中，钻孔之间的气压在最佳直径之前减小，而在最佳直径之后急剧增加。对于平煤八矿，最佳井眼直径约为1.0 m。现场测试表明，优化的HF可以大大消除煤与瓦斯突出的风险。